I was prescribed a medication which can increase blood pressure as a side effect, so I impulsively bought a small home BP measurement unit of the foolproof kind. It is made by a known brand for home medical electronics, so I think it's of reasonable quality. I've never operated one before, although I've had it used on me.
After extensive RTFM, I used it several times over two days, and I frequently got weirdly low measurements. Diastolic was mostly between 50 and 60, but a few times, I saw the display drop to 30, then turn off and give an error. Right now, half an hour after the medicine, diastolic was 63, but systolic was only 90.
Seeing that I usually have a BP in the middle of the healthy range (when measured by somebody who presumably knows what he's doing), and I don't feel close to fainting, I assume that I'm doing something wrong when measuring. I tried doing everything right - sitting straight in a chair, cuff on the naked left arm at heart height, not moving for a while. What are the possible error sources left? Could I be making the cuff too tight or too loose? I am velcroing it at a width which feels similar to a long-sleeve undershirt.